A great comparison can be found in Arizona, ironically at the Gabrielle Giffords assassination attempt.
While this was a Democrat rally, many AZ Democrats carry guns, and there were several at the rally with concealed carry. When the madman opened fire, two or three were getting ready to shoot him, but hadn’t a clear field of fire. And one of them was moving towards the madman with intent to shoot him.
But right then, the madman needed to pause to reload, so instead of shooting him, that cc holder tackled him.
In the French circumstance, the terrorists were likely firing on full automatic, that empties a magazine very quickly. This would give lots of time for someone with concealed carry firing a semi-automatic pistol, to put one or more aimed rounds in as many terrorists as he had bullets.
Additionally, I've seen and read eyewitness accounts that the shooters were all firing in unison rather than staggering their fire. This was notable to the eyewitness because she said there was a definite gap while they all reloaded. Personally, that's when I'd have made a move, very quickly, and gotten me some muslim a$$ to beat on...and a rifle. What the heck? You're gonna die anyway.